In recent years, development of hybrid vehicles and electric vehicles has been urged against the background of various problems related to the global environment, energy and the like. Motors mounted on these vehicles can achieve higher power by being driven with a higher voltage produced by a voltage booster circuit added in a stage preceding a motor drive power converter. Furthermore, there has recently been a growing demand that the performance of onboard power converters for motors be enhanced through higher-frequency switching.
Nowadays, as a boost converter, a multi-phase trans-linked boost chopper circuit is described in Japanese Patent Application Publication No. 2010-4704. The multi-phase trans-linked boost chopper circuit is capable of suppressing increase in switching loss, which occurs due to the higher-frequency switching, by suppressing the switching loss during diode recovery and in the turning-on of switches.
In the boost chopper circuit described in Japanese Patent Application Publication No. 2010-4704, a first switch is connected to the two ends of a DC power supply via a primary winding of a first transformer and a first reactor, and a second switch is connected to the two ends of the DC power supply via a primary winding of a second transformer and a second reactor. A first series circuit including a hoist winding of the first transformer, a first diode and a smoothing capacitor is connected to the two ends of a series circuit including the first reactor and the first switch. The hoist winding of the first transformer is connected to the primary winding of the first transformer in series. A second diode is connected to a connection point between the first reactor and the first switch, as well as to one end of the smoothing capacitor.
A second series circuit including a hoist winding of the second transformer, a third diode and the smoothing capacitor is connected to the two ends of a series circuit including the second reactor and the second switch. The hoist winding of the second transformer is connected to the primary winding of the second transformer in series. A fourth diode is connected to a connection point between the second reactor and the second switch, as well as to the one end of the smoothing capacitor. A third reactor is connected to the two ends of a series circuit in which a secondary winding of the first transformer and a secondary winding of the second transformer are connected together in series. A control circuit is configured to alternately turn on the first switch and the second switch at half-cycle intervals while keeping the first switch off during the ON period of the second switch, and the second switch off during the ON period of the first switch.
This configuration is capable of suppressing recovery loss in the first, second, third and fourth diodes, as well as switching loss in the turning-on of the first and second switches because: the first reactor is connected to the first switch in series; and the second reactor is connected to the second switch in series.
However, this configuration is still not capable of reducing switching loss in the turning-off of the switches. As the output from the power converter becomes larger, the switching loss which occurs when the larger power is converted by the switching becomes larger.